Eruption of a plasma blob, associated M-class flare, and large-scale EUV wave observed by SDO

We present a multiwavelength study of the formation and ejection of a plasma blob and associated EUV waves in AR NOAA 11176, observed by SDO/AIA and STEREO on 25 March 2011. SDO/AIA images clearly show the formation and ejection of a plasma blob from the lower solar atmosphere at ~9 min prior to the onset of the M1.0 flare. This onset of the M-class flare happened at the site of the blob formation, while the blob was rising in a parabolic path with an average speed of ~300 km/s. The blob also showed twisting and de-twisting motion in the lower corona, and the blob speed varied from ~10-540 km/s. The faster and slower EUV wavefronts were observed in front of the plasma blob during its impulsive acceleration phase. The faster EUV wave propagated with a speed of ~785 to 1020 km/s, whereas the slower wavefront speed varied in between ~245 and 465 km/s. The timing and speed of the faster wave match the shock speed estimated from the drift rate of the associated type II radio burst. The faster wave experiences a reflection by the nearby AR NOAA 11177. In addition, secondary waves were observed (only in the 171 \AA channel), when the primary fast wave and plasma blob impacted the funnel-shaped coronal loops. The HMI magnetograms revealed the continuous emergence of new magnetic flux along with shear flows at the site of the blob formation. It is inferred that the emergence of twisted magnetic fields in the form of arch-filaments/"anemone-type" loops is the likely cause for the plasma blob formation and associated eruption along with the triggering of M-class flare. Furthermore, the faster EUV wave formed ahead of the blob shows the signature of fast-mode MHD wave, whereas the slower wave seems to be generated by the field line compression by the plasma blob. The secondary wave trains originated from the funnel-shaped loops are probably the fast magnetoacoustic waves.Comment: A&A (in press), 22 pages, 13 figure

The Emergence of a Twisted Flux Tube into the Solar Atmosphere: Sunspot Rotations and the Formation of a Coronal Flux Rope

We present a 3D simulation of the dynamic emergence of a twisted magnetic flux tube from the top layer of the solar convection zone into the solar atmosphere and corona. It is found that after a brief initial stage of flux emergence during which the two polarities of the bipolar region become separated and the tubes intersecting the photosphere become vertical, significant rotational motion sets in within each polarity. The rotational motions of the two polarities are found to twist up the inner field lines of the emerged fields such that they change their orientation into an inverse configuration (i.e. pointing from the negative polarity to the positive polarity over the neutral line). As a result, a flux rope with sigmoid-shaped, dipped core fields form in the corona, and the center of the flux rope rises in the corona with increasing velocity as the twisting of the flux rope footpoints continues. The rotational motion in the two polarities is a result of propagation of non-linear torsional Alfv\'en waves along the flux tube, which transports significant twist from the tube's interior portion towards its expanded coronal portion. This is a basic process whereby twisted flux ropes are developed in the corona with increasing twist and magnetic energy, leading up to solar eruptions.Comment: 33 pages, 14 figures, Submitted to Ap

Shock wave diffraction phenomena around slotted splitters

In the field of aerospace engineering, the study of the characteristics of vortical flows and their unsteady phenomena finds numerous engineering applications related to improvements in the design of tip devices, enhancement of combustor performance, and control of noise generation. A large amount of work has been carried out in the analysis of the shock wave diffraction around conventional geometries such as sharp and rounded corners, but the employment of splitters with lateral variation has hardly attracted the attention of researchers. The investigation of this phenomenon around two-dimensional wedges has allowed the understanding of the basic physical principles of the flow features. On the other hand, important aspects that appear in the third dimension due to the turbulent nature of the vortices are omitted. The lack of studies that use three-dimensional geometries has motivated the current work to experimentally investigate the evolution of the shock wave diffraction around two splitters with spike-shaped structures for Mach numbers of 1.31 and 1.59. Schlieren photography was used to obtain an insight into the sequential diffraction processes that take place in different planes. Interacting among them, these phenomena generate a complicated turbulent cloud with a vortical arrangement

Free and forced wave propagation in a Rayleigh-beam grid: flat bands, Dirac cones, and vibration localization vs isotropization

In-plane wave propagation in a periodic rectangular grid beam structure, which includes rotational inertia (so-called 'Rayleigh beams'), is analyzed both with a Floquet-Bloch exact formulation for free oscillations and with a numerical treatment (developed with PML absorbing boundary conditions) for forced vibrations (including Fourier representation and energy flux evaluations), induced by a concentrated force or moment. A complex interplay is observed between axial and flexural vibrations (not found in the common idealization of out-of-plane motion), giving rise to several forms of vibration localization: 'X-', 'cross-' and 'star-' shaped, and channel propagation. These localizations are triggered by several factors, including rotational inertia and slenderness of the beams and the type of forcing source (concentrated force or moment). Although the considered grid of beams introduces an orthotropy in the mechanical response, a surprising 'isotropization' of the vibration is observed at special frequencies. Moreover, rotational inertia is shown to 'sharpen' degeneracies related to Dirac cones (which become more pronounced when the aspect ratio of the grid is increased), while the slenderness can be tuned to achieve a perfectly flat band in the dispersion diagram. The obtained results can be exploited in the realization of metamaterials designed to control wave propagation.Comment: 25 pages, 20 figure

3D MHD Flux Emergence Experiments: Idealized models and coronal interactions

This paper reviews some of the many 3D numerical experiments of the emergence of magnetic fields from the solar interior and the subsequent interaction with the pre-existing coronal magnetic field. The models described here are idealized, in the sense that the internal energy equation only involves the adiabatic, Ohmic and viscous shock heating terms. However, provided the main aim is to investigate the dynamical evolution, this is adequate. Many interesting observational phenomena are explained by these models in a self-consistent manner.Comment: Review article, accepted for publication in Solar Physic

Soft self-assembly of Weyl materials for light and sound

Soft materials can self-assemble into highly structured phases which replicate at the mesoscopic scale the symmetry of atomic crystals. As such, they offer an unparalleled platform to design mesostructured materials for light and sound. Here, we present a bottom-up approach based on self-assembly to engineer three-dimensional photonic and phononic crystals with topologically protected Weyl points. In addition to angular and frequency selectivity of their bulk optical response, Weyl materials are endowed with topological surface states, which allows for the existence of one-way channels even in the presence of time-reversal invariance. Using a combination of group-theoretical methods and numerical simulations, we identify the general symmetry constraints that a self-assembled structure has to satisfy in order to host Weyl points, and describe how to achieve such constraints using a symmetry-driven pipeline for self-assembled material design and discovery. We illustrate our general approach using block copolymer self-assembly as a model system.Comment: published version, SI are available as ancillary files, code and data are available on Zenodo at https://doi.org/10.5281/zenodo.1182581, PNAS (2018

MHD waves in sunspots

The review addresses the spatial frequency morphology of sources of sunspot oscillations and waves, including their localization, size, oscillation periods, height localization with the mechanism of cut-off frequency that forms the observed emission variability. Dynamic of sunspot wave processes, provides the information about the structure of wave fronts and their time variations, investigates the oscillation frequency transformation depending on the wave energy is shown. The initializing solar flares caused by trigger agents like magnetoacoustic waves, accelerated particle beams, and shocks are discussed. Special attention is paid to the relation between the flare reconnection periodic initialization and the dynamics of sunspot slow magnetoacoustic waves. A short review of theoretical models of sunspot oscillations is provided.Comment: 20 pages, 6 figures, Chapter in AGU Monograph (in press), Review articl